Multidisciplinary Senior Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: P18352
VACUUM ACTUATED SAFETY TRIGGER
Peter Bruschi
Diaphragm Lead, Team Communicator, Machinist
KGCOE, MECE
Steven Maniscalco
Project Manager, Lead Machinist
KGCOE, MECE
Aaron Neininger
Lead Designer, Lead Draftsman, 3D Printing
KGCOE, MECE
Robert Runk
Magnet Lead, Testing & Data Analysis Lead
KGCOE, MECE
ABSTRACT
The goal of this project is to design and test a pressure safety switch that is mechanical, highly reliable, withstands
rocket launch conditions, actuates in a near space environment, and fits within a given size constraint. The final design
was tested inside of a vacuum chamber and mounted to a shaker plate to simulate operating conditions. Upon
completion, the switch will be delivered to the customer, who will send the switch to space on a test rocket in the fall
of 2018 to verify function in a real-world scenario.
INTRODUCTION
A safety switch is a device used to stop or allow the operation of equipment under specific conditions. These
switches require high reliability because they prohibit a device from functioning unless the desired condition is met.
These devices are used in a number of applications, such as safeties on heavy machinery or firearms.
The customer is in need of a mechanical pressure safety switch for an asteroid deflection rocket. The switch must
trigger when the rocket breaches the atmosphere by sensing the low pressure environment. The switch behaves as a
safety to prevent energy from passing deeper into the system prematurely.
Although numerous electronic and digital systems already exist for measuring low pressures, the customer
requests that a purely mechanical system be developed for their application. This is because mechanical systems are
generally the most simple, secure, and reliable—where software or electronic systems are more complicated and prone
to glitches or hacking. Mechanical pressure sensing systems on the market today, such as piston-springs or
diaphragms, do not have the ability to read pressures as low as required by the customer, but were utilized as
benchmarks during the concept development process [1].
PROCESS/METHODOLOGY
The detailed customer requirements for the pressure safety switch are as follows:
Copyright © 2018 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
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The switch must be entirely mechanical for sensing the vacuum environment.
The switch must actuate after an absolute pressure of 10 -1 mm Hg has been reached.
The switch must withstand a maximum acceleration of between 20g and 27g from the rocket launch.
The switch should be easily resettable for testing purposes.
The switch must close a single electrical channel with a maximum resistance of 0.2 Ω. This electrical channel
must carry a load of 30 VDC with a 15Ω load in series down stream for a maximum of 15 seconds.
Once the electrical channel is closed, the channel must stay closed.
The switch must fit within 1/10th of a 13.6” diameter circle, by 6” tall.
The following assumptions were made to simplify the solution:
● Temperature change has a negligible effect on the system.
○ This is because in a real launch scenario, the rocket carrying the switch is expected to reach the
activation pressure in less than 1 minute. This gives little time for the temperature surrounding the
system to decrease and adversely affect normal operation.
● The Earth’s magnetic field has no effect on the permanent magnets contained within the system.
○ Magnetism depends on how atoms are oriented internal to a magnet. Travel through the atmosphere
should have no effect on the polarity of the system [2].
● The friction of the thin film diaphragm is negligible
Figure 1: Functional Decomposition of Vacuum Safety Switch
A functional decomposition was created to identify the appropriate subsystems of the device (Figure 1). A
morphological chart was then created to list all the different subsystems, and each team member came up with their
own individual ideas to complete each function. Systems utilizing valves, pistons, springs, and diaphragms were
selected based on research of mechanical pressure sensing equipment. The group selected the diaphragm concept
(Figure 2) using a decision matrix based on seven criteria: sensitivity, cost, size, resilience, resettability, potential
energy, and ease of manufacturing. It was agreed that the diaphragm over a sealed chamber would be the simplest
system to implement, with minimal moving parts to hinder sensitivity. A magnet system would also be implemented
to control the deflection of the diaphragm and the activation point of the switch.
Several materials were proposed for the diaphragm. These included nylon, rubber, and polyimide film (Kapton).
Each of the diaphragm materials were pressurized to 14-16 psi to simulate the pressure differential that will be
experience in the usage scenario (Figure 3). Kapton was found to have the most consistent deflection over 30 trials,
where the other materials would stretch or even burst under repeated fatigue. Additionally, Kapton has stable
properties across a wide range of temperatures, and has prevalent use in space applications [3].
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Proceedings of the Multi-Disciplinary Senior Design Conference
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Figure 2: Diaphragm-magnet system chosen for development
Figure 3: Diaphragm proof of concept
The feasibility of a diaphragm was analyzed by assuming a circular plate with a uniform load [4]. Using this
mathematical analysis, we find that a Kapton diaphragm would deflect over an eighth of an inch when subjected to
vacuum on one side, and atmospheric pressure on the other side (Figure 4). A drill bit was constrained to the center
of the diaphragm via a plug, and the deflection was measured to verify these calculations.
Figure 4: Validation of diaphragm concept
Copyright © 2018 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
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The pressure safety switch relies on a magnet-diaphragm system to sense and activate at the desired pressure
(Figure 2). As the ambient pressure outside of the chamber decreases, the air sealed inside the chamber will expand,
swelling the diaphragm and pushing the steel disk upward. A lever arm resting on the steel disc transfers the pressure
from the swelling diaphragm to the toggle switch. Once the activation pressure is reached, the lever arm will exert a
force that is greater than the friction of the toggle switch. This will force the switch to close, and allow the rocket
launch cycle to proceed to the next phase. To control the point of activation, the magnet system is put in place to slow
the swelling of the diaphragm and activation of the switch. One magnet is placed in the sealed chamber underneath
the diaphragm to pull the steel disk downward, counteracting the force of the pressure inside of the chamber. A second
magnet is placed over the top of the diaphragm, and is used for fine tuning of the activation point. This top magnet is
further away from the diaphragm than the bottom magnet, and is positioned to use a more gradual portion of the
magnet force curve. At the activation point, there will be 11.4 lbs of force on the diaphragm. Without magnets in
place, the system will activate around 600 mm Hg—6000 times higher the required activation point.
K&J Magnetics’ D9C magnet was selected to begin testing on different steel disks. The D9C is rated to apply
20 lbf to a steel plate [5]. The pull force on three different size steel disks was verified using an Instron 1125 Load
Frame to determine the best diameter disk for this application. The disks were attached to a non-ferrous holder, and
the load frame measured the force to pull the magnet off of the disk (Figure 5). Based on these results, the 5/8"
diameter disk was chosen, as it was the only disk shown to generate a force greater than the 11.4 lbs exerted on the
diaphragm. To get the required 11.4 lb pull force, the magnet has to be 0.003” from the disk. Disk diameters larger
than 5/8” were not considered in this test due to concerns that a larger disk would hinder the flexibility of the
diaphragm.
Figure 5: Magnet Testing
Testing was also performed to determine a toggle switch for our application. First, four different switches that fit
the electrical specifications were tested to see which switch actuated with the least amount of force. The test was
created by using a scale and a small block of aluminum. The switch was slowly depressed onto the scale, and the force
required to throw the switch was recorded. This test was vital to ensure that the switch does not apply too much
resistance on the diaphragm and hinder its ability to pop up as intended. The switch with the lowest and most consistent
force was incorporated into the design (Figure 6).
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Figure 6: CAD model showing cross section of final switch design
A CAD model of the proposed switch was designed and fabricated using the facilities in the RIT machine shop.
Since the team wished to machine most of the parts themselves, a simple and practical design was created to ensure
ease of manufacturing. This will translate well to the customer should mass production become a necessity.
Figure 7: Prototype in bell jar. Validation of diaphragm displacement at left, switch activation testing at right
The prototype was subjected to over 36 hours of testing in a bell-jar vacuum chamber provided by Dr. Mario
Gomes. This extensive testing ensured that the system was sealing and could activate at the required pressure. Shake
testing was also conducted in 3 axes at Delphi Incorporated to ensure that the device would not activate prematurely
due to the vibrations from the rocket engines.
RESULTS & DISCUSSION
Copyright © 2018 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
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The switch passed the 3-axis vibration tests conducted by Delphi. Unfortunately, facilities could not be acquired
to verify that the switch would not activate due to the 20-27 g’s that could be sustained during the rocket launch.
Laboratory testing has shown that the system can detect up to 96.7% vacuum, however this is still not precise
enough for this safety switch application. The pressure at this point is still 200 times higher than the 99.9% vacuum
target specified by the customer. Data shows that a setpoint of 0.263 on the bottom magnet and 0.585 on the top
magnet will activate at the goal of 0.1mm Hg (Figure 8). This matches the predicted curve from the magnet test data.
However, when tested in maximum vacuum, these settings resulted in no activation. Settings of 0.255 bottom and
0.455 top were selected for the launch test, as they produced the latest activation in the vacuum chamber.
Figure 8: Curves generated from vacuum chamber testing
The team ran into several issues during the vacuum experiments. The switch would either activate too early during
the cycle, or not activate at all. Possibilities for the switch not activating could be attributed to issues with sealing, or
that the bottom magnet was too close to the diaphragm. This would make for too great of a force that the diaphragm
would have to overcome.
Since the bottom seal had to be broken in order to adjust the bottom magnet, the team believes that this could
have caused some inconsistency between experiments that made it difficult to find the ideal set point. The team
discussed further developments that could be made to the system, such as removing the bottom magnet adjuster and
implementing a magnet shim to achieve the proper clearance between the diaphragm and the magnet (Figure 8). The
team also discussed switching to a larger magnet to work in a more gradual region of the magnet pull curve, which
would give more resolution. Several shims were tested based on the larger diameter K&J Magnetics DCA, but the
team found that the larger diameter magnet did not improve the system as hoped. A longer magnet would be prefered
because of the higher flux density coming out of the top. This would give the required force and a more sloped force
curve, making the device easier to adjust. The new magnet would also make the required shim thickness larger and
easier to print or machine.
Figure 8: Proposed Redesign of Bottom Magnet Interface
Table 1: Analysis of requirements met by final switch design
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CONCLUSION
The safety switch fails to activate as low as 0.1 mm Hg, however, this is the only tested requirement that was not
satisfied (Table 1). Since all of the other requirements were achieved with great success, the team believes that this
magnet-diaphragm concept still could be utilized as a basis for further development by the customer. To improve the
system, stronger magnets will need to be used to increase the resolution of the system, and shim adjustment could be
implemented to eliminate issues with sealing, difficult adjustment, and consistency. The team anticipates that this
would help to achieve the remaining 2-3% of vacuum that the current switch is not precise enough to detect. The team
would continue on these developments if time permitted. The final switch will be set to activate as late as possible
during the real-world test launch.
Copyright © 2018 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
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REFERENCES
[1] Jousten, K, 2006, “CAS - CERN Accelerator School and ALBA Synchrotron Light Facility : Course on
Vacuum in Accelerators,” Platja d'Aro, Spain, pp.65-86 (CERN-2007-003)
https://cds.cern.ch/record/1046852/files/p65.pdf
[2] Sten Odenwald, 2006, NASA: IMAGE Education and Public Outreach, https://image.gsfc.nasa.gov/poetry/
[3] “Dupont Kapton (Polyimide Film),” MIT Material Property Database,
http://www.mit.edu/~6.777/matprops/polyimide.htm
[4] “Circular Plate Uniform Load Edges Simply Supported,” Engineers Edge,
https://www.engineersedge.com/material_science/circular_plate_uniform_load_13638.htm
[5] “DC9,” K&J Magnetics, https://www.kjmagnetics.com/proddetail.asp?prod=D9C
ACKNOWLEDGEMENTS
Team VAST would like to thank the following people and organizations for their support throughout this project:
Tom Bitter, for his brilliant mentoring and endless enthusiasm;
Nick Leathe and Sandia National Lab, our customer contact and sponsor for the VAST system development;
Professors Gomes, Humphrey, and Hanzlik, for design and testing advice, work space, and technical assistance;
Scott Blondelle, for vacuum equipment and technical assistance;
Michael Abbatte and Delphi Corporation, for conducting vibration testing and generating reports;
KGCOE Machine Shop Staff, for fabrication advice and support;
MSD Staff, for making this opportunity possible.
Project P18352